$obj = $t->create_ok('/url', { some => 'object' });
$obj = $t->create_ok('/url', 'filename');
$t->create_ok('/url', <many/files*>);
if called with a filename, loads the object from the file as
described in testdata().
Uses POST to the url to create the object, encoded with JSON.
Checks for status 200 and returns the decoded body.
You can also create multiple objects/files at once, but then there is
no returned object.
This counts as 2 TAP tests.